Abaixo está uma instrução que descreve uma tarefa de geração de SQL, combinada com uma entrada que fornece mais contexto sobre os esquemas de tabela disponíveis. Escreva o código SQL que responda adequadamente à solicitação
 ###Instrução: qual é a média de prata quando o total é superior a 20?
 ###Tabelas: CREATE TABLE investment_and_financing (investment_and_financing_id AUTO_INCREMENT PRIMARY KEY, Department VARCHAR, Projects INT, Municipalities INT, Farmers INT, Irrigated Ha INT, BID/PRONAR Investment (US$) VARCHAR, Local Investment (US$) VARCHAR, Total VARCHAR) CREATE TABLE economies_of_member_states (economies_of_member_states_id AUTO_INCREMENT PRIMARY KEY, Member State sorted by GDP VARCHAR, GDP in s billion of USD (2012) VARCHAR, GDP % of EU (2012) VARCHAR, Annual change % of GDP (2012) VARCHAR, GDP per capita in PPP US$ (2012) INT, Public Debt % of GDP (2013 Q1) VARCHAR, Deficit (-)/ Surplus (+) % of GDP (2012) VARCHAR, Inflation % Annual (2012) VARCHAR, Unemp. % 2013 M7 VARCHAR) CREATE TABLE d_&_r (d_&_r_id AUTO_INCREMENT PRIMARY KEY, Serial & Branch INT, Seaman VARCHAR, Mechanical VARCHAR, Secretariat VARCHAR, Supply VARCHAR, Electrical VARCHAR, Radio Electrical VARCHAR, Regulating VARCHAR, Medical VARCHAR) CREATE TABLE 2009_nfl_draft (2009_nfl_draft_id AUTO_INCREMENT PRIMARY KEY, Round INT, Pick # INT, Player VARCHAR, Position VARCHAR, Height VARCHAR, Weight VARCHAR, College VARCHAR, Status VARCHAR) CREATE TABLE medal_count_by_country (medal_count_by_country_id AUTO_INCREMENT PRIMARY KEY, Rank INT, Nation VARCHAR, Gold INT, Silver INT, Bronze INT, Total INT)  
SELECT AVG Silver FROM medal_count_by_country WHERE Total > 20